The present invention relates to a highly reliable continuously variable transmission (CVT) component, such as a disk and a power roller bearing for use in a toroidal-type continuously variable transmission (CVT), and a method for evaluating the same.
Since CVT disk and power roller are used under the high load and high surface pressure environment and are ranked as important safety parts, in order to prevent them against breakage and exfoliation in a short time of use, as the material of the CVT disk and power roller, there is used high-purity steel for a rolling bearing. In such high-purity steel, in order to extend the life of the rolling bearing as well as to enhance the reliability of the rolling bearing, the particle diameter distribution of oxide-system inclusions (the number and size of the inclusions) in a give area (or volume) to be sampled must be limited; and, therefore, for such limitation, there are proposed a large number of inclusion detect methods, for example, in Japanese Patent Unexamined Publication No. Hei. 3-56640, Japanese Patent Unexamined Publication No. Hei. 7-109541, Japanese Patent Unexamined Publication No. Hei.5-117804, and Japanese Patent Unexamined Publication No. Hei. 6-192790.
Conventionally, as an ultrasonic defect detect method for detecting the internal defects of a bearing ring, there is known a vertical defect detect method in which, in a process for manufacturing steel material for a bearing ring, ultrasonic waves are transmitted through the steel material after rolled from the outer peripheral surface thereof to the interior portion thereof under water or on a table to thereby detect the defects of the bearing ring (Special Steel, Vol. 46, No. 6, Page 31 published by Special Steel Club (Corporation)
By the way, although the high-purity steel for a rolling bearing is manufactured in a highly controlled line in order to remove non-metallic inclusions which give rise to the defects of the high-purity steel, actually, it is impossible to remove all of such defects. And, even in the high-purity steel, once in a great while, there can suddenly occur a defect and it is impossible to prevent such sudden occurrence of the defect completely. Especially, a large-size inclusion is fundamentally not allowed to exist in the vicinity of the surface of the high-purity steel because it gives rise to the bending fatigue rupture of the high-purity steel in a CVT disk. For this reason, it is necessary to check all of the CVT disks and power rollers for the defects thereof.
However, it is a main object of the above-mentioned conventional defect detect method to detect internal defects present in blow holes formed in the interior portion of the steel material or in the unrolled portion of the steel material left in the rolling process thereof, not to detect defects in the vicinity of the surface of the steel material, especially, large-size inclusions of the order of several hundred μm. This is partly because the surface of the steel material is rough just as it is rolled and thus, just below the surface of the steel material, an insensitive zone is large for the ultrasonic defect detect method, and also partly because the steel material includes bent and distorted portions and thus it is difficult to keep constant the distance between the steel material and a detect probe, especially, it is impossible to detect a micro-defect in the vicinity of the surface of the steel material. Also, in case where the interior portion of steel material having a large diameter of more than 100 mm is checked for the defect thereof, detecting frequencies must be lowered so as to prevent the sensitivity from lowering due to the attenuation of ultrasonic waves, so that nothing but the large-size defect having a size of several mm or so can be detected.
On the other hand, the main object of an ultrasonic defect detect method for a rolling body is to detect a surface flaw and a defect in the upper-most or extreme surface portion of the steel material with high efficiency. For example, using ultrasonic waves having high frequencies such as 50–150 MHz, a microscopic non-metallic inclusion having a size of up to 0.01 mm (10 μm) can be detected.